Testing tubes filled with a chemical reagent are used to determine gaseous components in gas mixtures that are aspirated or pressed through the testing tube. The reaction taking place with the powdered or granulated reagent produces a reaction zone marked by a change in color. The length of this zone is proportional to the quantity of the gas component which has flowed into the testing tube and to which the reagent specifically responds. If the quantity of the gas mixture passing through the testing tube is known, then the proportion of the gas component to be detected, or its concentration, can be ascertained from the length of the reaction zone.
German Pat. No. 2,840,867 discloses a gas measuring device wherein the migration of the reaction zone in the testing tube is ascertained by means of a photoelectric scanner and is specified as a measured quantity via a receiver connected thereto. The testing tube is uniformly illuminated by a light source, and the light radiation reflected by the reaction zone is determined in a receiver via fiber optics with light conductors. This kind of measuring device affords monitoring of the advancing length of the reaction zone without having any moving parts, and it enables electrical measurement or control signals to be derived which not only indicate the concentration of the component that is to be detected, but also trigger an alarm device in the event that predetermined limit values are exceeded